The invention concerns a method and a device for producing microdroplets of fluid.
In a great number of chemical or physical processes, particularly in drying and combustion processes, it is of great importance to obtain reactive microdroplets of fluid. Ordinarily, a fluid is pressed to this end through a specially designed atomizer nozzle, which effects a spraying apart or an atomizing of the fluid. The atomizing can also be effected with the aid of steam or compressed air, whereas these methods are not used with small amounts of fluid.
It is also generally known how to improve or accelerate the exit of a fluid jet from a nozzle through a gas flow surrounding the emerging jet in concentrical manner. However, the gas flow is not intended to effect an atomization of the fluid emerging from the nozzle but rather, on the contrary, to hold together the jet of the fluid. Finally, it is also known how to convey a rotary movement to the thin gas mantle holding together the fluid jet or also a droplet sponge, in order thereby to obtain a rotation of the fluid jet proper (DE-OS No. 1 475 162). However, also with this known solution the intention is to avoid an atomization of the fluid or further fine atomization.
The present invention is now based on the problem of creating a method and a device for producing microdroplets of fluid, which method or which device permits an extremely fine atomization also at a very low fluid pressure.
This problem is solved with regard to the method of the invention in that
a fluid is injected from an opening into an atomizer chamber in such a manner that a substantially hollow spray cone is formed and that PA0 this spray cone is acted upon by an external gas flow, the flow path of which is approximately concentric and spiral-shaped in relation to the theoretical axis of the spray cone, so that the spray cone is broken up by the flow of the gas.
In accordance with the invention a violent collision of the fluid and the flow of gas is brought about intentionally and in controlled manner. Thereby it is possible to obtain also a fine atomization at a very low pressure of the fluid emerging from the opening. A maximum fine atomization is obtained with the method of the invention even with very small amounts of flow.
Preferably, the radius of the spiral-shaped path of the flow of the gas in the direction away from the opening, through which the fluid is injected into the atomizer chamber, is reduced to an ever increasing extent at a uniform rate, is possible. Thereby the flow of gas experiences an additional acceleration, with the consequence that the droplets of fluid carried along, are broken up to an increasing extent. Extremely fine droplets of fluid or microdroplets of fluid are obtained in the order of magnitude of about 20 .mu.m. Such a small mean size of droplets cannot be obtained with the known atomizer nozzles or methods. In most instances a reduction of the mean size of the droplets to a level below 50 .mu.m was unsuccessful due to the limited possibilities of manufacturing technology available. There are spray nozzles for such a coarse dispersion with nozzle slots uniformly distributed over the periphery having a width of about 100 .mu.m each. Since manufacturing tolerances between 98 .mu.m and 102 .mu.m cannot be avoided, such spray nozzles result in an uneven distribution of the spray or an uneven distribution of the droplets. In addition, it was shown that spray slots having a width of about 100 .mu.m can be easily and quickly clogged up when fluids are used which contain solid particles (impurities), for example oil, after a short time, when such fluid is used for atomizing. Subsequently, after having been used for a longer period of time the distribution of the droplets is nonumiform. Impurities may lead to wear and tear which, in its turn, results in a nonuniform distribution. To further reduce the size of the droplets it has been shown that it is advantageous to introduce the droplets of the fluid through an opening into a preferably cylindrical transport chamber and to carry them by way of a spiral-shaped flow of gas to the end opposite to the inlet opening, which end is preferably open.
It is known that with the aid of a flow of gas it is possible to carry droplets of fluid from one point to another along a usually rectilinear path, where the section of conveyance is dimensioned in such a way that the droplets react chemically during their movement along this section, or experience a physical transformation, for example evaporation. The solution suggested by the invention now offers the advantage that the mentioned reactions can take place over a relatively short structural length of the transport chamber. Precisely with combustion units it is of special importance to obtain an overall compact installation.
By means of the last mentioned solution an extremely long transport section is obtained for the droplets of the fluid carried along by the flow of the gas, by way of a chamber, the structural design of which is relatively short. This makes it possible, for example, to bring droplets of the fluid in a very small "reaction chamber" or transport chamber to a complete evaporation. The method according to the invention is particularly suitable for drying and burning fluids, because it is generally known that with smaller droplets a drying or combustion process takes place faster and is more complete. The dependence between processing time t (time of drying or combustion) and diameter of the droplets d is as follows: EQU t=c.multidot.d.sup.1.8,
where c is a constant. The processing time t is the period of residence required in the transport or reaction chamber, where due to the path of movement of the droplets in the transport chamber as provided by the invention, this time limit can also be met with a very small transport chamber.
In most cases it must be avoided that the droplets of the fluid in the atomizer chamber and/or transport chamber or reaction chamber come into contact with the inner surface of the chamber walls. Corresponding deposits on the inner surface of the chamber walls should be avoided. In order to achieve this, the gas is introduced into the atomizer chamber and/or transport chamber advantageously at a distance from the inner surface of the chamber walls.
In order to obtain a still greater fineness of the droplets of the fluid, the gas can be given a spinning or rotary movement of its own along the path of the flow. Then the flow of the gas is characterized by two superimposed rotary movements.
In accordance with the apparatus aspects of the invention, an apparatus for producing microdroplets of fluid broadly comprises a small tube the outlet opening of which is generally centrally located within an atomizing chamber, and a plurality of gas inlet passages radially spaced from the small tube opening and adapted to impart a spiral-shaped motion onto gas introduced into said atomizing chamber through the gas inlet passages. Preferably the cross-section of the atomizing chamber decreases in the direction of flow towards the outlet of the atomizing chamber, such decrease desirably being uniform.
In accordance with still further apparatus aspects of the invention, the apparatus may further comprise a transport chamber having at one end thereof an inlet opening in flow communication with the outlet of the atomizing chamber, and a plurality of gas entry openings radially spaced from the inlet opening of the transport chamber and adapted to impart a spiral-shaped motion onto gas introduced into the transport chamber through the gas entry openings; the other end of the transport chamber being preferably open. In one form of the apparatus, at least one of the gas entry openings is provided at the one end of the transport chamber, and guide plates are provided for deflecting the gas to impart the spiral motion thereto. In another form of the invention, at least one gas entry opening is formed by a bore extending in an oblique manner to the radial line of the transport chamber in a lateral wall forming a lateral boundary of the transport chamber. Desirably a tube is inserted within the bore so as to project beyond the inner surface of the lateral wall whereby contact of fluid droplets introduced into the transport chamber may be reduced.